In conventional systems the percent consistencies off the drums of a pulp washing system are not measured because there has been no known method by which to measure them except by hand sampling and lab testing. The operators run by experienced observation only and no measurements of the percent consistency are routinely made and no target percent consistency is ever established for the operator even as a guide. The percent consistencies off the drums are recognized as important variables but no means have been available to continuously measure and control them.
The normal operating procedure is usually one of the following four methods.
A. The operator uses his experience of observation to control the drum speed and vat dilution such that the pulp mat being formed "looks" about right. Changes are made to these variables periodically as rate changes are made in the incoming pulp to the system or as quality factors such as drainage rate of the pulp change. There is no measurement or control of the discharge percent consistency of the pulp on any of the drums in this control method. The vat dilution flow is not measured but the drum speed is generally indicated.
B. In some systems the operator sets the vat dilution flow by experience to achieve a reasonable vat percent consistency by visual observation and the drum speed is then set on automatic control by an instrument measuring the liquid level in the vat. The vat dilution flow is not measured but the operator knows by experience what valve opening to set on the automatic valve regulating the vat dilution flow. The hydraulic capacity of the drum is proportional to the drum speed but not linearly and if the liquid level rises then the instrument automatically increases the speed of rotation of the drum to bring the level back to the control point. Again, there is no measurement or control of the discharge percent consistency off any of the drums with this method of control even though the vat percent consistency and drum speed both affect the percent consistency of the discharged pulp.
C. The third method of control is less prevalent but is very similar to the second method except that the operator sets the speed of rotation of the drums and the level control instrument then raises or lowers the vat dilution flow to maintain the vat level. As in the two previous methods there is no measurement or control of the discharge percent consistency off any of the drums and also generally no measurement of the vat dilution flow rate. The drum speed is however generally measured.
D. The fourth method of control is described in U.S. Pat. No. 4,840,704 entitled "Controlling Characteristics of a Pulp Mat on a Pulp Washing Surface" whereby the drum speed and vat dilution on the last washer in the line is controlled by use of a mass measuring gauge or similar device on that washer and the object of the patent is to control the total weight per unit area off that one washer drum to a constant value to facilitate the accuracy of the total mass measurement, which is non-linear with respect to the thickness of the pulp mat, and to maintain a relatively constant percent consistency in the vat of the washer at various tonnage rates. This method, like the other three methods, does not measure or control the percent consistency of the pulp leaving any of the washer drums in the pulp washing system.
These methods all fail to determine the percent consistency off any of the drums in a series of drums in a pulp washing system and cannot optimize any of the variables described herein. Additionally, with these methods of control it is not necessary to measure the actual flow of the showers to any except the last stage of washing and therefore these measurements are not made.
In U.S. Pat. No. 4,207,141, to the author of this application (Seymour), a method is described to determine the consistency of the pulp leaving the last stage of washing by using a combination of a capacitance measuring instrument and a total mass measuring instrument but this system was never implemented as no such capacitance device was ever practical.
Following are descriptions of the various parameters that affect the discharge percent consistency of the pulp leaving each individual washer and thereby affect the overall efficiency of the washing process.